1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to wireless communication systems.
2. Description of the Related Art
FIG. 1 illustrates a portion of a geographic area in which wireless services are provided by a first service provider having a first network of cells 10 and a second service provider having a second network of cells 20. Each of the cells in the first and second network of cells 10 and 20 represent a coverage area for a corresponding base station in the network run by the respective service provider. A base station generally provides wireless communication services for mobile stations 12 within its coverage area.
As will be appreciated, the communication between a base station and a mobile station 12 has great potential for interfering with the communication between another base station and another mobile station 12. This potential interference exists for base stations in the same network as well as between base stations in different networks. Also, communication between a single base station and two different mobile stations 12 has the potential for interfering with one another. To avoid this interference, wireless communication systems such as Code Division Multiple Access 2000 (CDMA 2000) wireless systems use codes to differentiate transmissions. As will be described below, base stations BSs use codes generated from different offset values to differentiate between transmissions by different base stations. Also, a base station BS uses codes to differentiate between different traffic channels assigned to mobile stations 12 by the base station BS.
In a CDMA 2000 system, each service provider assigns an identifier BS_ID to the base stations to uniquely identify each base station in the network. However, a base station identifier BS_ID does not necessarily uniquely identify base stations in different networks. When transmitting, a base station codes its transmission based on a different offset such as the pseudoNoise (PN) offset in CDMA 2000. In CDMA 2000, the PN offset is a 9-bit string that differentiates one base station BS from its neighboring base stations even when those base stations are in different networks. As illustrated in FIG. 1, base stations from different networks may be neighboring base stations. While FIG. 1 illustrates neighboring base stations as having coverage areas that are adjacent or near one another (e.g., separated by one or more cells), it will be understood that neighboring base stations may also have overlapping coverage areas. The mobile station 12 obtains the PN offset from the base station to decode the transmissions from the base station and differentiate the transmission of one base station from another base station. In CDMA 2000, for example, the mobile station 12 obtains the PN offset as well as the base station identifier BS_ID from a message broadcast by the base station over a forward control channel.
When a base station assigns a traffic channel over which the mobile station 12 may communicate, the base station uses codes to differentiate one traffic channel transmission from another traffic channel transmission. In CDMA 2000, a different Walsh code is used to code each traffic channel of a base station. As with the PN offset, the Walsh code assigned to a traffic channel between the base station and a mobile station 12 is communicated to the mobile station 12 in a message over a forward control channel. At the mobile station 12, the mobile station 12 decodes the transmission over the traffic channel and differentiates one traffic channel from another using the Walsh code.
Codes are also used to differentiate reverse link transmissions (mobile station to base station) over traffic channels. The code used by a mobile station to code and differentiate traffic channel transmissions is referred to as a code mask. In a CDMA 2000 system, the code mask is referred to as the long code mask, and two types of long code masks are known: private and public. In a CDMA 2000 system, the public long code mask (PLCM) is typically formed using the electronic serial number (ESN) of the mobile station 12. The PLCM in a CDMA 2000 system is 42-bits long; however, other long code mask sizes exist. Typically, the PLCM includes a plurality of bits for indicating the type of the long code mask (private or public), along with a 32-bit ESN of the mobile station 12. The 32-bit ESN is often considered the variable portion of the PLCM.
For the base station to decode the traffic channel transmission for the mobile station 12, the base station needs to obtain the mobile station's ESN, and numerous methods are known in the art for accomplishing this. As an alternative to the above-discussed PLCM generation method, a CDMA 2000 system also provides that the base station BS may assign a PLCM of its choosing to a mobile station 12 during a call activation (e.g., call origination or call delivery).
Owing to the large and growing number of mobile stations 12, the supply of 32-bit ESNs is being exhausted, and discussions have begun on transitioning from the use of 32-bit ESNs to 56-bit mobile equipment identifiers (MEIDs). The use of a 56-bit MEID poses several challenges in the context of generating public long code masks. Directly using a 56-bit MEID value (for a PLCM that is otherwise 42-bits long) would require hardware changes. Accordingly, several proposals exist for using the MEID to generate the public long code mask that does not require such hardware changes. One proposal is to map the 56-bit MEID to a 24-bit value, concatenate a fixed 8-bit value to the 24-bit mapped value and create a 32-bit pseudo-ESN. The pseudo-ESN could then be used as the ESN in the conventional public long code mask generation process.
However, it has been shown that this pseudo-ESN method does not result in a sufficient number of different public long code masks to prevent an undesirable number of collisions between mobile station communications. A collision occurs when the transmissions from two or more mobile stations 12 are not uniquely identifiable.
Other techniques have been proposed to resolve this collision problem, including one in which PLCMs are assigned based on the longitude and latitude value of the base station. In this base station-assigned PLCM proposal (hereinafter referred to as “BSAPLCM” proposal), all of the base stations use only one fixed format that supports all cell sizes, both large and small. In the BSAPLCM proposal, and as shown in FIG. 2, the PLCM is formed of 42-bits, where the first 3 bits are utilized to distinguish between private/public code masks and to differentiate between the MEID generated PLCM and BS-Assigned PLCM. The remaining 39 bits of the PLCM are associated with three different components or fields—latitude (11 bits), longitude (11 bits), and a mobile station ID (MS_ID) (17 bits). The “longitude” and the “latitude” fields respectively contain a longitudinal value and a latitudinal value that represent the position of the base station. These values are expressed in units of “x” seconds, and are converted to an 11-bit number. The conversion to an 11-bit number is described in greater detail below. The mobile server ID field is a 17-bit field that is assigned by the base station to uniquely identify the mobile units 12. The longitude, latitude, and mobile server ID fields collectively make up the remaining 39 bits of the PLCM.
The BSAPLCM proposal suggests that a given base station can be identified to a precision of substantially 0.25 seconds if 22-bit and 23-bit values are used to respectively represent the latitude and the longitude information associated with that base station. However, using a combined 44-bit value to represent the latitude and longitude information is not feasible because of the restriction imposed by the 42-bit long PLCM value. The proposal suggests using, for example, 11 bits to represent the latitude value and 11 bits to represent the longitude value of a base station, as shown in FIG. 2. The BSAPLCM proposal arrives at the 11-bit longitude and latitude values by right shifting the corresponding 23-bit longitude and 22-bit latitude values by 5 bits and then converting the remaining bits to 11 bits using a MOD operation.
Reducing the longitude and latitude fields from the respective 23 and 22 bits to 11 bits naturally results in a loss of resolution in terms of the precision by which a particular base station can be distinguished from another at a given location. For example, according to the BSAPLCM proposal, shifting the higher-bit values by 5 bits results in a 0.15 mile granularity. That is, the base stations located within 0.15 miles of each other will have identical 11-bit longitude and latitude values. Thus, the base stations within this range cannot be distinguished from each other. Furthermore, the proposal acknowledges that reducing the number of bits employed to represent the longitude and latitude values exacerbates this issue, as two stations located 314 miles apart will have the same 11-bit longitude and latitude values. The proposal refers to the distance at which the base stations may have the same 11-bit longitude and latitude values repeat as the “reusable distance” (e.g., the 314 miles in the example discussed above).
The BSAPLCM proposal suggests that problems of multiple base stations that are located in close proximity (e.g., 0.15 miles) to each other, and thus have the same 11-bit longitude and latitude value, can be alleviated by partitioning the mobile station IDs (see FIG. 2) among these base stations. Thus, even though a PLCM value may be calculated based on the same 11-bit longitude and latitude values associated with the two different base stations, the rationale is that the mobile station IDs will be unique to each user. This approach involves some manual intervention by the service provider.
The BSAPLCM proposal suffers from at least one shortcoming in that its calculations for the reusable distance (e.g., ˜314 miles) are based upon conditions on or about the Earth's equator. That is, the proposal does not take into account the fact that the distance between longitudinal lines decrease as one moves away from the equator. As the distance between the longitudinal lines decrease, the reusable distance similarly decreases. A reduced reusable distance, however, is undesirable because the potential for collisions is greater. That is, as the reusable distance decreases, it becomes more likely that a mobile station 12 may obtain a PLCM in a first region and then travel the reusable distance to a second region where the same PLCM has been assigned to another mobile station 12. Accordingly, an improved way of generating PLCMs is desired to reduce the potential of collisions.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.